© 2021 Acoustical Society of America.Parabolized stability equations (PSE) have been shown to model wavepackets and, consequently, the near-field of turbulent jets with reasonable accuracy. In this work, PSE were employed to obtain a reduced-order model that could estimate both the fluid-dynamic and the acoustic fields of a supersonic jet in a computationally efficient approximation for resolvent-based estimation based on a single input. From the unsteady pressure data at an input position, the time-domain pressure field was estimated using transfer functions obtained using PSE and a data-driven method based on a well-validated large-eddy simulation (LES). The prediction scheme employed is a single-input single-output, linear model. The unsteady pressure predicted by the PSE showed good agreement with the LES results, especially if the input position is outside the mixing layer, where the prediction capabilities of the PSE are comparable to those of the data-driven transfer functions. The good agreement indicates that PSE could not only be used to predict the sound generation but also to open up different potentialities to attenuate the noise by flow control. The exploration of the regions where the method displayed good agreement, which are presented in this work, can guide the positioning of the sensors for experimental implementation of closed-loop control in a jet.

© Published under licence by IOP Publishing Ltd.Floating offshore wind turbines (FOWTs) are the next frontier in offshore wind energy, allowing exploration of deep-water regions previously unavailable to fixed-foundation turbines. Since offshore turbines operate in lower turbulence levels, the intrinsic hydrodynamic unstable modes of the tip vortices can have even more relevance than in onshore turbines. For floating turbines, platform motion induced by wind and wave loads can trigger vortex instabilities, modifying the wake structure, possibly influencing the flow reaching downstream wind turbines. In the present paper, we study those effects by the means of numerical simulations and their comparison with analytical studies. In our simulations, the wind turbine blades are modeled as actuator lines in the incompressible Navier-Stokes equations. Heave motion with different amplitudes and frequencies are studied. The effect of increasing amplitude is to advance the onset of vortex interaction. For the lower frequency of heave motion, several vortices coalesce to form a large flow structure. High amplitude of oscillations in the streamwise velocity were observed due to these flow structures, which may increase fatigue or induce high amplitude motion on downstream turbines. The number of vortices that interact, as other qualitative phenomena of the numerical simulation, were well predicted by a simple stability model of two-dimensional row of vortices. The disturbances imposed by the heave motion were also compared to the eigenvectors resulting from linear stability theory for helical vortices and the predicted growth rates for the wavenumbers resulting from this comparison were consistent with the model of a row of vortices. These results motivate further studies to understand the impact of the larger flow structures on downstream turbines.

© 2019 IOP Publishing Ltd. All rights reserved.The hydrodynamic stability of a vortex system behind two in-line wind turbines operating at low tip-speed ratios is investigated using the actuator-line method in conjunction with the spectral-element flow solver Nek5000. To this end, a simplified setup with two identical wind turbine geometries rotating at the same tip-speed ratio is simulated and compared with a single turbine wake. Using the rotating frame of reference, a steady solution is obtained, which serves as a base state to study the growth mechanisms of induced perturbations to the system. It is shown that, already in the steady state, the tip vortices of the two turbines interact with each other, exhibiting the so-called overtaking phenomenon. Hereby, the tip vortices of the upstream turbine overtake those of the downstream turbine repeatedly. By applying targeted harmonic excitations at the upstream turbine's blade tips a variety of modes are excited and grow with downstream distance. Dynamic mode decomposition of this perturbed flow field showed that the unstable out-of-phase mode is dominant, both with and without the presence of the second turbine. The perturbations of the upstream turbine's helical vortex system led to the destabilization of the tip vortices shed by the downstream turbine. Two distinct mechanisms were observed: for certain frequencies the downstream turbine's vortices oscillate in phase with the vortex system of the upstream turbine while for other frequencies a clear out-of-phase behaviour is observed. Further, short-wave instabilities were shown to grow in the numerical simulations, similar to existing experimental studies [1].

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.An experimental investigation has been undertaken to study the wavy leading edge phenomena on rectangular wing of aspect ratio 4 with a NACA 0020 airfoil at Reynolds number range from 700,000 to 3,000,000. Force measurements for various shapes of sinusoidal leading edge indicate smaller amplitude and shorter wavelength configuration (A3λ11) presenting a substantial increase in aerodynamic performance at entire range of Reynolds number tested when compared to baseline configuration, as result achieving 28.3 % of increasing in maximum lift coefficient. Oil flow visualizations reveal that tubercles with smaller amplitude have the role of delaying trailing edge flow separation. At high angles of attack, the A3 λ 11 configuration is shown to present spanwise wavelengths for which the optimal generation of streaks in turbulent boundary layers is expected according to previous experimental works. The appearance of such streaky boundary layers is a possible reason for the delay in flow separation and increase of maximum lift coefficient.

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.The swept wing configuration showed interesting results for the leading edge phenomena in recent studies. The most important outcomes showed that configurations with higher wing loading at the tip (such as tapered and swept wings), tend to be more susceptible to the effect of tubercles, giving an important increase in stall angle, and consequently, in maximum lift coefficient. A remarkable gain of about 20% on CLmax (without large penalties on drag coefficient) was obtained in 30° swept models with taper ratios of 1 and 0.5.This increase in lift force is associated to the fact that the wavy configurations exhibit a delay on stall progression from wing tip to the root as consequence of the tubercle effects, which maintain the flow attached on leading edge at high angles of attack. To investigate the relationship between the increase in maximum lift coefficient and the swept angle, and continue with the study presented by Abrantes et al.11, a series of experiments were conducted on six wing configurations. The range of sweep-angles include wings with 30°, 40° and 50°, as well as two variations of taper ratio (TR=1 and TR=0.5). All models had an underlying NACA 0020 airfoil and a wavy geometry with amplitude A=0.03c and wavelength λ=0.11. The purpose of this research consisted in evaluating the drag and lifts forces at Reynolds number of 200,000 for all seven models and compares their results with the smooth wings. In addition, a flow visualization analysis using oil technics was included in order to better understand the involved phenomena.

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.The study of wavy leading edge phenomena on finite wings conducted on recent research has shown results that demonstrate improvements in aerodynamic characteristics for certain configurations. The most important outcomes were observed on swept and swept-tapered wings, attaining improvement in lift coefficients of around 20% when compared with their equivalent baseline models. This increase on lift force is associated to the fact that the wavy configurations exhibit a delay on the stall angle due to the effect of the tubercles, which main effect is to delay the stall progression from tip to root by keeping the flow attached on the leading edge at high angles of attack. Visualization results confirmed larger effect of these phenomena on the wingtip area. In order to give continuity to previous works that investigated swept wing with wavy leading edge, obtain a deeper knowledge of this phenomena and delimit the design space in which the wavy leading edge could be efficiently applied, a series of experiments were conducted on sixteen wing configurations including swept angles of 30 and 50 degrees, taper ratio of 1 and 0.5, and wavy span length of 20, 40 and 100% (from tip to root). All models had an underlying NACA 0020 airfoil and a wavy geometry with amplitude A = 0.03 and wavelength λ = 0.11 considering the root chord as reference. The purpose of this research consist in evaluating of drag and lift forces at Reynolds number of Re = 200, 000 for all models showing comparative results with the baseline wings. In addition, a flow visualization analysis using oil technique was included in order to better understand the involved phenomena.

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.Parabolized Stability Equations (PSE) have been shown to model wavepackets and, consequently, the near field of turbulent jets with reasonable accuracy. Because of these capabilities, PSE is a promising reduced-order model to derive control laws that could be employed to reduce the sound generation of a jet. The purpose of this work is to apply PSE to obtain time-domain transfer functions that could estimate both the fluid-dynamic and the acoustic fields of a supersonic jet. The results of this model were compared to results obtained from a database of a well-validated large-eddy simulation of a supersonic jet. Based on the unsteady pressure data at a input position, the time-domain pressure field was estimated using transfer functions obtained using PSE and an empirical method based on the LES data. The prediction scheme employed is a single-input-single-output (SISO), linear model. The unsteady pressure predicted by PSE showed good agreement with the LES results, especially if the input position is outside the mixing layer. For this region, the prediction capabilities of PSE are comparable to those of empirical transfer functions. The agreement is good even for output points taken in the acoustic field, showing that it is possible to estimate the time-domain behaviour of Mach-wave radiation using transfer functions. This indicates that PSE could not only be used to predict the sound generation, but also to open up new potentialities to attenuate noise by means of closed-loop control of the flow. The exploration of the regions where the method displayed good agreement, presented in this work, can guide the positioning of sensors and actuators for experimental implementation of closed-loop control in a jet.

© 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.A distinct wavy leading edge performance for finite wings can be expected in comparison to the infinite wing, due to the differences in geometry and flow conditions. An infinite span wing, unlike a partial span model, has a unique local Reynolds number, sweep angle, thickness and camber. In addition, it is not subjected to the wing tip phenomenon which changes the pressure coefficient along span, and, as consequence, the adverse pressure gradients. These differences on the flow over finite and infinite span geometries cause differences in tubercle performance, which have motivated some works, in order to investigate the influence of flow three-dimensionality on wavy leading edge performance. However, there are lack of works that evaluate the effects of the wing’s three-dimensional on flow topology of the wavy leading edge and their consequences in performance. The aim of this study is to investigate the effects of the wing’s three-dimensional flow on wavy leading edge phenomena at low Reynolds number. Experimental investigations were carried-out modifying geometric parameters of the wing planform (taper ratio and sweep) in order to understand the effects of these parameters on wavy leading edge phenomena. The tests are conducted for pairs of models with and without tubercles. A pair of two-dimensional models (NACA 0020) and four pairs of finite-wing models with taper ratios of 0.5 and 1, and sweep angles of 0º and 30º were tested. The experimental investigation was based on evaluation of force measurements (lift and drag) and flow visualizations (oil and mini-tufts). Additionally, the Reynolds number effects were also investigated by evaluating the wavy leading edge characteristics at Reynolds number 80,000 and 200,000.

© 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.The interest in studying the aerodynamic performance of airfoils at low Reynolds number has been increasing recently. There are many applications for airfoil design at low Reynolds number as design point. These applications include sailplanes, propellers, unmanned aerial vehicles (UAVs) and micro air vehicles (MAVs). However, there are few works evaluating the thickness effect as a design parameter for airfoil performance at low Reynolds number regime as well as data from flow visualizations in order to clarify the aerodynamic phenomena regarding thickness variation. In this sense, the aim of this work is to investigate the thickness effect on flow characteristics and performance of symmetrical airfoils at low Reynolds number regime by experimental investigation correlating force measurements with mini-tuft and oil flow visualization data. In an overall view, this work intends to contribute for investigations of desirable flow and geometric conditions of airfoils applied in UAV and MAV designs. Experimental tests were carried out at subsonic blower-type wind tunnel of open loop with closed section at ITA (Technological Institute of Aeronautics). A set of three symmetrical airfoils with different thickness (NACA 0012, NACA 0020 and NACA 0030) were tested at Reynolds number regime between 50,000 and 290,000. The results show distinct thickness effect for Reynolds number condition borders where at Re = 50,000 the thickest airfoil causes full flow separation with a great aerodynamic deterioration. In contrast, at Re = 290,000 the thickest airfoil achieves the highest maximum lift value.

© 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.The aim of this work is to investigate the wavy leading edge phenomena for the very thick airfoil NACA 0030 by experimental investigation correlating force measurements with mini-tuft and oil flow visualization data at low Reynolds number regime. Experimental tests were carried out at subsonic blower-type wind tunnel of open loop with closed section at ITA (Technological Institute of Aeronautics). A set of four very thick airfoils were tested composed by one smooth configuration and three wavy leading-edge configurations (A= 0.03c, λ = 0.40c; A= 0.03c, λ = 0.11c; A= 0.11c, λ = 0.40c). The wavy leading edge geometry variation and Reynolds number effects were evaluated at range of Reynolds number between 50,000 and 290,000. For the highest Reynolds number condition, the results show worse aerodynamic performance for wavy leading edge when compared to previous studies of thinner airfoils. However, at Reynolds number 120.000, the configuration with A= 0.03c and λ = 0.11c presents a unprecedented result on literature overcoming the baseline maximum lift coefficient in 19,4% and the stall angle in 44%. In addition, the flow visualization results indicate that the leading edge stall characteristics at airfoils lead the tubercle configurations for better aerodynamic performance.